Add ensure_array_for_loops feature for XML-to-JSON compatibility

[bene-tyler]

Add opt-in environment flag to automatically wrap non-array values in arrays when used in for loops. This solves the common XML-to-JSON cardinality problem where single items are objects but multiple items are arrays.

Problem solved:

• XML with one converts to object: {"person": {...}}
• XML with multiple converts to array: {"person": [{...}, {...}]}
• Previously required different templates or custom ensureArray() callback or is_array() if statement
• Now no custom code is needed to use a for loop with json objects that change cardinality per request.

Usage:
env.set_ensure_array_for_loops(true); // {% for p in person %} now works whether person is object or array

Implementation:

• Added RenderConfig::ensure_array_for_loops flag (defaults to false)
• Added Environment::set_ensure_array_for_loops() method
• Created ensure_array() helper function that wraps non-arrays:
  • Arrays: returned unchanged
  • Null: becomes empty array []
  • Objects/primitives: wrapped in single-item array [value]
• Modified ForArrayStatementNode::visit() to call ensure_array() when enabled

Testing:

• 26 new test assertions in test-ensure-array.cpp
• Tests single objects, arrays, null, primitives, nested loops
• Tests default behavior, enabled behavior, and disabled flag
• All 284 assertions pass (26 new + 258 existing)
• No regressions in existing functionality
• Stress tested with 10,000 iterations

Backward compatibility:

• 100% backward compatible - flag defaults to false
• Without explicit enablement, behavior is identical to previous versions
• Non-arrays still throw errors by default
• Existing code requires zero changes

Documentation:

• Added usage example to README.md
• Comprehensive test suite demonstrates all use cases